This invention relates to the production of fuels from waxy, paraffinic hydrocarbons produced by the reaction of CO and hydrogen, via the Fischer-Tropsch hydrocarbon synthesis process. In particular, this invention relates to a diesel fuel and a process for its production, useful as a winter diesel fuel derived from wax containing Fischer-Tropsch feeds utilizing a multi-zone isomerization process to yield diesel fuels having both excellent low temperature properties and emissions performance.
Upon combustion in a vehicular diesel engine, diesel fuels are known to emit undesirable pollutants, for example; solid particulate matter (PM) e.g., soot, as well as gaseous pollutants, e.g., unburned hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). This is especially true during the winter months, as emissions consisting of HC, NOx, CO and particulates are increased due to the cold temperatures. In addition, cold weather operation places a greater strain on the emissions performance of the fuel, especially the cold-start and smoke performance. As a result, seasonal winter diesel fuels are required in much of the world during these cold periods in order to reduce and/or control emissions. In addition to increased environmental standards, winter diesel fuels need exceptional low temperature flow properties, Accordingly, there have been numerous government and industry efforts to design winter fuels for use in the mogas/diesel markets to address the winter emissions quandary. The current results are winter diesel fuels which are traditionally blended with, or consist essentially of kerosene and are characterized by improved cold flow properties.
Fischer-Tropsch fuels are well known as high cetane number fuels. However, due to the highly paraffinic nature of Fischer-Tropsch fuels, these fuels generally possess mediocre to poor cold flow properties, i.e., cloud point and cold filter plugging point. The heaviest paraffin molecules tend to crystallize as wax particles and precipitate above certain temperatures, resulting in high freeze point and/or cloud point. This makes it difficult or even impossible to use such products in many environments. Improving the cold flow properties of these fuels generally leads to decreasing the cetane number as is the case with conventional fuels. As a result, there is a need in the mogas/diesel markets for a high cetane, low cloud/cold filter plugging point (CFPP) diesel which exhibits emissions performance better than that which would be expected from the currently documented government and industry correlations for fuel effects on emissions.
In accordance with an embodiment of this invention, is provided a novel process for the production of a fuel, useful as a winter diesel fuel, derived from Fischer-Tropsch products, which meets cold flow standards while simultaneously providing superior emissions performance. The process comprises separating the paraffinic, wax containing Fischer-Tropsch product into a 300xc2x0 F.+ distillate fraction and upgrading the fraction via hydroisomerization and selective catalytic dewaxing to produce a diesel fuel with excellent low temperature properties and reduced emissions.
In particular, a 300xc2x0 F.+ (149xc2x0 C.+) fraction derived from the Fischer-Tropsch process is passed into a first reaction zone, of two sequential isomerization reaction zones in a single reaction stage, the first reaction zone comprising a first catalyst containing a suitable hydroisomerization catalyst, to form a first zone effluent. At least a portion of the liquid product from the first zone effluent, preferably the entire liquid product from the first zone effluent, is passed into a second reaction zone, comprising a second catalyst having a catalytic dewaxing functionality, to form a second zone effluent. In the alternative, the second reaction zone may contain a mixture or composite comprising both catalytic dewaxing and hydroisomerization catalysts. The first and second zones may be in the same or separate reaction vessels and preferably both zones are contained in the same reaction vessel. Further, the first and/or second reaction zone may comprise one or more catalyst beds. The second zone effluent comprises an isomerized hydrocarbon product and can be fractionated into desired liquid product fractions, e.g., a 320-700xc2x0 F. boiling fraction.
By 300xc2x0 F.+ fraction is meant the fraction of the hydrocarbons synthesized by the Fischer-Tropsch process and boiling above a nominal 300xc2x0 F. boiling point. At least a portion of the product of the second reaction zone is recovered to produce a middle distillate boiling in the diesel fuel range, i.e., a 320-700xc2x0 F. boiling fraction. Preferably, the process is conducted in the absence of intermediate hydrotreating, and produces products with excellent cold flow characteristics, i.e., cloud and freeze point, superior smoke point and better than expected emissions characteristics. Such distillate products can be used as diesel fuels or as blending components therefor. Preferably, this isomerized product of the second zone effluent produces a fuel having a T95 (the temperature at which most all the material has boiled oft, leaving only 5% remaining in the distillation pot) greater than or equal to 240xc2x0 C. (464xc2x0 F.) but less than or equal to 350xc2x0 C. (662xc2x0 F.), preferably a T95 greater than or equal to 245xc2x0 C. (473xc2x0 F.) but less than or equal to 340xc2x0 C. (644xc2x0 F.), more preferably a T95 greater than or equal to 245xc2x0 C. (473xc2x0 F.) but less than or equal to 335xc2x0 C. (635xc2x0 F.) and having a cold filter plugging point as determined by IP-309 below about xe2x88x9235xc2x0 C., more preferably below about xe2x88x9240xc2x0 C. and contains:
In one embodiment the invention comprises a Fischer-Tropsch hydrocarbon synthesis process, in which a synthesis gas comprising a mixture of H2 and CO, react in the presence of a suitable Fischer-Tropsch hydrocarbon synthesis catalyst, to form a hydrocarbon product wherein a 300xc2x0 F.+ distillate fraction and hydrogen or a hydrogen containing gas is passed into a first reaction zone, having a first catalyst comprising a suitable hydroisomerization catalyst to form a first zone effluent and the total liquid product from the first zone effluent is passed into a second reaction zone having a second catalyst containing either a catalytic dewaxing catalyst, a hydroisomerization catalyst or a mixture thereof, to form a second zone effluent comprising an isomerized hydrocarbon product. A middle distillate fuel, useful as a diesel fuel is recovered from the hydrocarbon product of the second reaction zone.
In this way, good yields of distillate fuels with excellent cold flow properties are produced from wax containing paraffins derived from the Fischer-Tropsch process to produce a full boiling range diesel fuel, preferably a 320-700xc2x0 F. fraction, with the unique combination of high cetane number, very low cloud and cold filter plugging point (CFPP) performance and full boiling range cut exhibiting superior emissions performance.
Optionally, all or part of the hydrocarbon product recovered from the second reaction zone can be combined or blended and used as a winter diesel fuel. When used as a blend, the product of this invention can be used in relative minor amounts, e.g., 10% or more, for significantly improving the final blended diesel product